install compiler
$ sudo port install arm-none-eabi-gcc arm-none-eabi-binutils
checkout sourcecode
$ git clone -b pico https://github.com/raspberrypi/micropython.git
$ cd micropython
$ git submodule update --init -- lib/pico-sdk
$ cd lib/pico-sdk
$ git submodule update --init
$ cd ../..
build
$ make -C mpy-cross
$cd ports/rp2
$ make